I started using silk plants a few months ago, as I was unable to find aquarium-specific plants that fit my themes. (I've done african safari, light houses, cherry blossom trees/asian, frog pond, and "two roads in a yellow wood", so I wanted something different than the usual underwater plants readily available.) So, I've dealt with this problem. :)
Some silk plants can be purchased with thick, plastic stems that don't have metal in them. These are perfect for crafting trees or other things you want to "stand up" in the tank. You can attach leaves or flowers or whatever else you like on there with hot glue (works for a bit, is non-toxic, but will eventually need to be reglued) or with aquarium sealant (which should hold them more permanently, and is also non-toxic).
Also, to create layers of foliage in your tank, you can tie them to the sides (clip them on with clothes pins or even tape them to the outside of the tank), allowing whatever length of plain ribbon you want to determine how high up you want the plants to float. Same with suction cups. Just suction cup the plant to the side of the tank. I've used narrow silk ribbon in white, and never had a problem with it leaching dye or anything into the tank.
Also, some terrarium plants are actually cheaper, give more foliage, and all are aquarium safe, so you can get more "bang for your buck" in terms of plants. And, many of these come with their own suction cups and stick on the sides.
One other thing...and I'm sure I may get some people jumping on me for saying it...but I have, on occasion, left the metal "stems" in the plants. IF you do this, make *certain* that the metal is completely enclosed in plastic or aquarium sealant, so it's not rusting and leaching nastiness into your water. Also, I've ONLY done this with "stems" and not any part of the plant that will be places where betta will be swimming or resting in. (IE, I'll leave the metal in the "trunk" of the tree, removing it from all "branches", so the only stiff part of the plant is that which is going straight up and down, and is not something that betta can get trapped between, or where he's going to be sleeping or swimming *through*, only around.) Mine have never had any issues with this, no torn fins, anything of that nature.
Also, if you use store-bought silk plants, make sure to wash them well, and to soak them for some time to make sure that the dye isn't going to leach out, and that they're safe for fishies. :)
Good luck to you, and I hope this helps!! :D